LEPUS CUNICULUS 331 



bands, the ventral pyramids, lying one on each side of the shallow 

 median ventral fissure. Outside these at the front end, that is, 

 immediately behind the pons, are to be distinguished a pair of areas 

 termed the corpora trapezoidea. Posteriorly the medulla passes on 

 into the spinal cord without any sharp line of demarcation. 



There is but one cavity common to both the metencephalon and 

 myelencephalon, and that is the fourth ventricle. 



Cranial Nerves. 



Twelve large pairs of nerves leave the cranium in the rabbit, 

 and they are commonly termed the cranial nerves, but, strictly 

 speaking, one of them, the eleventh or spinal accessory, is largely 

 composed of fibres coming from the spinal cord. The points of 

 origin of these nerves, while they can be made out in the rabbit, are 

 much better studied in a larger brain, and they will, therefore, be 

 considered later, although it should be borne in mind that they arise 

 in the same relative position in both rabbit and sheep, and, indeed, 

 the first ten are constant throughout the chordate series from dog- 

 fish to man himself. It should be borne in mind that although we 

 speak of ten cranial nerves in lower vertebrates and twelve in the 

 higher forms, there is present in all of them from lowest to highest 

 another nerve right at the anterior end. This is termed the nervus 

 terminals, and it arises from the front median portion of the telence- 

 phalon and passes forward in the neighbourhood of the olfactory 

 lobe to be distributed in the region of the internasal septum. Its 

 exact functional significance has not yet been ascertained, though 

 it is present in all forms. Again, we find in all vertebrates a second 

 small nerve, the vomero-nasalis or septalis, which arises not far 

 from the other, but developmentally just behind the olfactory. 

 This, too, is distributed in the neighbourhood of the nasal septum, 

 and its function is not yet understood. So that to be strictly 

 accurate we should say there are twelve cranial nerves in the lower 

 Chordates such as Rana and Scy Ilium, and fourteen in the higher 

 forms like Lepus. 



We may now consider briefly the distribution of these 

 nerves in the rabbit. 



The nervus terminalis is a small nerve, comprising only a few 

 fibres, passing out on the mesial wall of the cerebral hemisphere in 

 the sagittal fissure. It runs through the cribriform plate with the 

 fibres of the olfactory nerve, and spreads out to help to form a 

 plexus, in company with the nervus septalis, in the neighbourhood 

 of the nasal septum and Jacobson's organ. 



The olfactory nerve, composed of numerous branches, ramifies 

 all over the olfactory membrane in the nasal chamber and covering 



